The coloration of anodized aluminium for decorative and aesthetic purposes in architectural applications has been a permanent need for over 40 years.
Initially the system used was COLORATION BY IMPREGNATION of the porous anodic film with organic or mineral pigments. The greatest disadvantage of these systems was the lack of stability of the colours to atmospheric exposure.
Another very old coloration system is INTEGRAL COLORATION. Such is essentially based upon the use of aluminium alloys containing certain intermetallic elements or compounds, insoluble in the electrolyte used in the anodizing process. During formation of the anodic film the intermetallic compounds are trapped inside the same, originating a limited range of gold, bronze, grey and black colours.
The films produced using this system are extremely hard, with an excellent resistance to corrosion. The colours obtained are also very strong to sunlight.
This aluminium coloration system however poses a number of problems, in particular as follows:
In order for the colour to be uniform, a very precise control is required in preparing and homogenizing the alloy, and later transforming the same, i.e. at the extrusion or lamination stages. PA1 A very precise control of the anodizing electrolyte is also required. PA1 Voltages much greater than those used in conventional anodizing are required. Consequently, energy consumption is far greater, and may be calculated to be 5 to 10 times greater than in conventional anodizing, obviously rendering this system almost inadmissible. PA1 Colour intensity is intimately linked to the thickness of the film obtained.
The above problems per se indicate the scarce practical interest of this alumnium coloration system.
The system of METALLIC ELECTROLYTIC COLORATION of anodized aluminium appeared towards the end of the nineteen sixties. In these processes, coloration is obtained by deposition and accumulation of metallic particles from the bottom of the pores towards the surface portion of the anodic film.
The colour is produced by different optical effects, namely refraction, deflection, absorption and internal reflection of light, falling on and crossing the transparent anodic film.
The incidence of light on the surface of the metallic deposit barely causes preferential absorption of the electromagnetic waves of the visible spectrum. Almost all metals produce a slightly yellowish colour, saving transition metals such as copper which further yield orange and reddish colours.
On increasing the side surface of the metallic deposit, the internal reflections are multiplied, thereby to increase diffuse reflection and hence internal absorption of all the electromagnetic waves of the visible spectrum. This leads to a shaded darkening of the yellowish colour, yielding a brown colour which has actually been designated bronze, and can even be a black.
This coloration system currently produces a limited range of gold, bronze and black colours. Although copper deposition can yield a range of reddish colours, this technique is rarely used because of the potential risks of corrosion it entails. The quality and stability of these finishes is optimal.
In the mid-nineteen seventies, a new technique of electrolytic coloration came to light, whereby it was possible to obtain new colours. This technique was actually designated ELECTROLYTIC COLORATION BY OPTICAL INTERFERENCE. U.S. Pat. Nos. 4,066,816, 4,251,330 and 4,310,586 describe different techniques of this coloration system.
The theoretical explanation of the process given in such patents is the following:
When a beam of white light falls on an anodic film a part of it is reflected and the other part crosses it, and its path is deviated due to a refraction effect.
A part of the beam crossing the anodic film is again reflected on falling on the metallic deposit, located at the bottom of the pores. The other part of the beam crosses the anodic film to arrive at the surface of the metal where it is reflected.
When separation between the plane defined by the upper surface of the metallic deposit and that of the aluminium surface acquires certain values, optical interference effects, constructive or destructive, can come about, and give rise to some of the colours of the visible spectrum.
The optical interference effect produced when a beam of light falls on and crosses a thin transparent film in a medium with a different refractive index is a known fact, described in any elementary optics text. (Francis Weston Sears. Principles of Physics Series. OPTICS. CHAPTER 8: INTERFERENCE. 8.1. INTERFERENCE IN THIN FILMS, page 203).
U.S. Pat. Nos. 4,066,816, 4,251,330 and 4,310,586 on electrolytic coloration by interference basically claim an effect and the conditions in which the same takes place which have been known for many years.
Without questioning the legal validity of the said patents, they are at fault from a theoretical standpoint, as follows:
They consider the layer delimited by the metal surface and an imaginary parallel surface comprising the upper part of the metallic deposit a thin layer. This layer is obviously discontinuous, being entirely different to the area taken up by the pores, where metallic particles are deposited, and not the porous portion constituted by aluminium oxide. It is difficult to imagine that the area between pores shall have a different refractive index to the rest of the anodic film and furthermore, if such were to be the case, that the said area would be perfectly distinct in a parallel plane from the metal surface (essential conditions for the interference effect to be produced).
Obviously, no optical interference can come about in the area of the layer taken up by the metallic deposit, for the white light cannot cross the metal and can only be more or less anarchically reflected, to cause a diffuse reflection.
The technique developed according to the theoretic model described in the above patents allows some colours of the visible spectrum to be obtained, preferably a bluish grey. From the practical standpoint the process poses huge repetitiveness and uniformness difficulties and has not therefore been widely applied industrially.